This invention relates to a field-effect transistor (abbreviated as FET) which employs an organic semiconductor. More particularly, it relates to a field-effect transistor for use as a switching device or as part of a drive circuit for a large-area liquid crystal display.
A .pi.-conjugated polymer has a structural skeleton consisting of conjugated double bonds or conjugated triple bonds. It is thought that it has a band structure consisting of a valence band formed by overlapping .pi.-electron orbits, a conduction band, and a forbidden band which separates the two. The width of the forbidden band varies depending on the material, but in most .pi.-conjugated polymers, it is 1.5-4 eV. For this reason, many .pi.-conjugated polymers are inherently electrical insulators. However, if so-called doping is performed in which electrons are removed from the valence band (resulting in oxidation) by chemical, electrochemical, or physical methods, or electrons are injected into the conduction band (resulting in reduction), it is thought that carriers which carry electrical charge are formed. As a result, by controlling the amount of doping, the electrical conductivity of a .pi.-conjugated polymer can be varied over a wide range from that of an electrical insulator to that of a metal. When doping is performed by oxidation, a p-type .pi.-conjugated polymer is obtained, and when doping is performed by reduction, an n-type .pi.-conjugated polymer is obtained. This process is analogous to the introduction of impurities into an inorganic semiconductor. Thus, by means of the above-described doping process, a semiconductor device using a .pi.-conjugated polymer as a semiconductor material can be manufactured.
A Schottky junction employing polyacetylene (Journal of Applied Physics, Volume 52, page 869, 1981; Japanese Patent Application Laid-Open No. 56-147486) and a Schottky junction employing a polypyrrole-type conjugated polymer (Japanese Patent Application Laid-Open No. 59-63760) are known. Also, a heterojunction element which is a combination of the inorganic semiconductor n-CdS and p-type polyacetylene has been reported (Journal of Applied Physics, Volume 51, page 4252, 1980). A junction which is a combination of two .pi.-conjugated polymers has been reported in the form of a p-n homo-junction employing p-type and n-type polyacetylene (Applied Physics Letters, Volume. 33, page 18, 1978). Furthermore, a heterojunction made from polyacetylene and poly(N-methylpyrrole) has been reported (Journal of Applied Physics, Volume 58, page 1279, 1985).
An FET employing a .pi.-conjugated polymer as a semiconductor layer was disclosed in the Journal of Applied Physics, Volume 54, page 3255, 1983 (using polyacetylene) and in Polymer Preprints, Japan, Volume 34, No. 4, page 917, 1985 (using poly(N-methylpyrrole)).
FIG. 1 is a cross-sectional view of a conventional FET employing polyacetylene as a semiconductor layer. In the figure, a gate 2 in the form of an aluminum layer or the like is formed on a substrate 1 made of glass or the like. An electrically-insulating film 3 comprising polysiloxane is formed on the substrate 1 so as to cover the gate 2. A semiconductor layer 4 which is made of polyacetylene is formed atop the electrically-insulating film 3, and a source 5 and a drain 6 in the form of gold films are formed atop the semiconductor layer 4 on both ends thereof.
The operation of the illustrated FET is as follows. When a voltage is applied between the source 5 and the drain 6, a current flows between the source 5 and the drain 6 through the polyacetylene semiconductor layer 4. If at this time a voltage is applied to the gate 2, the conductance of the semiconductor layer 4 can be slightly changed by the field effect. Accordingly, the source-drain current can be controlled. It is thought that the reason for this phenomenon is that the width of the depletion layer within the polyacetylene semiconductor layer 4 is changed in accordance with the voltage which is applied to the gate 2, and the effective channel cross-sectional area of holes is changed.
However, this type of FET has the problem that the polyacetylene which forms the semiconductor layer is subject to sudden degradation due to oxygen and moisture in the air, and therefore it has extremely poor stability.
FIG. 2 is a cross-sectional view of another type of conventional FET which employs poly(N-methylpyrrole) as a semiconductor layer. In this figure, an electrically-insulating film 3 comprising silicon dioxide is formed on a p-type silicon wafer 7 which serves as both a substrate and a gate. Two gold films, which serve as a source 5 and a drain 6 and are separated from each other are formed atop the electrically-insulating film 3 thereof. The source 5 and the drain 6 and the surface of the electrically-insulating film 3 therebetween are covered by a semiconductor layer 4 in the form of a poly(N-methylpyrrole) film. In the same manner as for the FET of FIG. 1, the current which flows between the source 5 and the drain 6 through the semiconductor layer 4 can be controlled by the voltage which is applied to the gate 7, which controls the conductance of applied to the semiconductor layer 4.
However, in an FET which employs a .pi.-conjugated polymer such as polyacetylene or poly(N-methylpyrrole) only as a semiconductor layer, only a moderate variation in the conductance between the source and the drain can be achieved by changing the gate voltage, and there is accordingly a need for an FET having improved characteristics.